Past studies on the mechanism of electron transfer by c-type cytochromes have yielded considerable information and have led us to formulate a number of interesting and potentially significant questions, including: the role of the protein surface structure and protein dynamics in electron transfer. The ultimate goal of this work remains to understand the mechanism of coupling biological electron transfer to energy conservation. However, to fully understand this process a number of related issues must be addressed which bear on both biological electron transfer and the general areas of prot ein structure and function. Thus, an integral part of elucidating the mechanism of biological electron transfer focuses on the chemical and structural aspects of the interaction between two proteins which give these reactions their high specificity and rapid rates. The studies proposed here are designed to address both the question of the mechanism of biological electron transport and the nature of protein-protein interactions. Specifically we propose to investigate the inter-actions of cytochrone c with non-heme iron sulfur proteins and photosynthetic reaction centers. We also are proposing experiments designed to elucidate the surface structure and dynamics of cytochromes in solution and to use this information in understanding protein-protein interactions and electron transfer. We anticipate that information derived from these studies will bear not only the mechanism of biological electron transfer but will provide information useful in understanding the general role of dynamics in protein function and the nature of specific chemical and physical inter-actions in reactions involving the interaction of macromolecules.